The present invention relates to video processing systems for altering video images of a sequence of images delivered by any video source (including, but not limited to, a camera, output of a studio switchboard combining multiple cameras, or satellite down or up links), for replacing a zone (hereinafter referred to as a target) of the scene in the images, by a matching representation of a predetermined pattern. The word "frame" will frequently be used in place of "image" in the following for more clarity and avoiding ambiguities. However, processing will only occur frame-to-frame when sequential raster scanning is used. When, in most cases, conventional interleaved raster scan is used, a full frame consists of two fields with a time delay between corresponding lines in the two fields of 20 ms in Europe and 16.67 ms in the U.S. Then, either of two approaches should be selected. One of the approaches consists in processing odd field separately from the even fields. However, it is much preferable to make an evaluation of the amount of movement which occur between two fields and to use interpolation techniques for obtaining an image whose lines correspond to the condition of the scene at a same instant.
A number of techniques which enable substitution of a pattern for a target are known. Reference may for instance be made to international application WO 91/15921, the content of which is included in the present specification by way of reference. Another method, disclosed in U.S. Pat. No. 5,107,252 (Traynar et al) requires video image manipulation by repeatedly pointing at the corners of the representation of the target to be substituted on a frame-by-frame basis. U.S. Pat. No. 5,263,933 (Rosser et al) teaches using conventional pattern recognition and image processing methods for recognizing a selected portion in a display, for later insertion of a video representation of stored indicia in place of that portion. Frame-by-frame processing involving manually designating a key frame to be substituted is time consuming and tedious. Any method requiring a full pattern recognition process carried out on each image has a number of limitations. For instance, the pattern recognition algorithm will not be able to identify a billboard to be substituted until and unless a significant portion of that billboard is present in the image. That may result in sudden substitution of the billboard with another billboard after a large portion of the billboard has entered the field of the camera, which is highly objectionable.